Hans-Joerg Fuchs

Highly efficient transmission gratings in fused silica for chirped-pulse amplification systems

We report on highly efficient transmission gratings in fused silica with a grating period of 800 nm generated by electron-beam lithography. At a wavelength of 1060 nm, 95% diffraction efficiency is achieved under Littrow conditions. The damage threshold, extremely enhanced compared with conventional gold-coated diffraction gratings, makes these gratings the key elements in high average power (>100 W) femtosecond fiber chirped-pulse amplification systems.

Highly efficient polarization-independent transmission gratings for pulse stretching and compression

Many industrial and scientific applications require high power ultrashort laser pulses, so amplification of pulses is necessary. To avoid optical damage or nonlinear effects in the amplifier setup, the pulses are stretched before amplification and recompressed afterwards. One possibility for the efficient stretching and recompression is to apply highly efficient diffraction gratings, whereby dielectric gratings and especially dielectric transmission gratings feature a high damage threshold. If the incident pulses are not linear polarized, the polarization sensitive diffraction efficiency of the gratings mostly results in a significantly reduced pulse energy. To overcome this problem we developed highly efficient polarization independent gratings and present a theoretical and experimental study on the design and fabrication of highly dispersive transmission gratings in fused silica, that exhibit a high diffraction efficiency for TE and TM-polarized illumination as well. The dependence of the diffraction efficiency on the grating parameters is discussed for both polarization directions. One of the theoretical designs shows a diffraction efficiency exceeding 97% for unpolarized illumination. The fabrication of those gratings has been done by electron beam lithography and reactive ion beam etching, whereby the diffraction efficiency was maximized by a special trimming process. The theoretical considerations are confirmed by the fabricated samples.

Fabrication technique for high-aspect-ratio gratings

High aspect ratio gratings are of interest for a couple of applications. Especially artificial birefringence based on zero order dielectric gratings are calling for small pitches (e.g. < 450 nm) and a high aspect ratio (e.g. > 10 in fused silica). One of the most difficult problems is to reach the optical parameters that are demanded. Based on e- beam writing, ion beam etching, reactive ion beam etching and chromium coating we developed a technique for the fabrication of such gratings. In result, we reached phase retardation between TM and TE polarization of more than 90 degree(s). An iterative step technique allows realization of the phase retardation of accuracy better than 1%. The highest aspect ratios of the gratings, we fabricated, were in the range of about 25 at 440 nm period and 100 nm gap.

Polarizing metal stripe gratings for a micro-optical polarimeter

Several applications in science and industry require a precise measurement of the polarization of the incident light, e.g. the online characterization of textile-fiber birefringency in spinning machines. A promising concept to realize these polarimeters is based on structured polarizers that consist of three polarizing metal stripe gratings, whose spatial orientations are tilted against each other with an angle of 60deg. With such an approach the state of polarization can be determined with a very small device without any moving parts. Here we report on the fabrication of a micro optical polarimeter based on three polarizing metal stripe gratings with a grating period of 300nm and a size of 0.3 mm x 0.3 mm each. To achieve the necessary high aspect ratio we combined electron-beam direct writing with dry etching and metal coating under oblique incidence. The measured polarization contrast of the fabricated micropolarizers is more than 300:1 over a large spectral range (wavelength between 450nm and 1000nm), moreover they have a contrast of even more than 1000:1 if light with a wavelength between 500nm and 600nm is used. Thus, they are well suited for microsensors to detect birefringency.

Fabrication of glass lenses by melting technology

The paper shows a fabrication technology for glass micro lenses. It starts with the fabrication of a boron phosphorus silicate glass (BPSG) layer on a quartz substrate. Pre-forms of BPSG are fabricated using photolithography and dry etching. After that, temper process at a temperature in between the melting temperature of each material changes the surface to the minimal surface. Spherical, cylindrical and even ball lenses are profiles which can be realized. Spherical, cylindrical, and ball lens profiles that can be produced.

Experimental realization of a diffractive optical isolator

We report on the realization of an diffractive optical isolator for use at 543 nm by the combination of two binary high frequency gratings, corrugated into the surface of a quartz substrate. A single-order grating acts as a polarizing beam splitter with a measured diffraction efficiency of greater 95%. The other grating is a zero-order diffraction with 290 nm period and 1300 nm depth, acting as a quarterwave plate for conical incidence. A good correlation between theoretical and experimental results is demonstrated.

Characterization of chirped phase masks for UV applications by writing and investigation of fiber Bragg gratings

We describe the characterization of chirped phase masks made by electron beam writing and reactive ion etching in pure silica. The phase masks are designed to be used as diffractive optical elements in the process of manufacturing of fiber Bragg gratings (FBG). With the technologies available at our institutes grating structures with periods less than 200 nm can be realized in fused silica. Phase masks for writing ideally chirped FBG would require grating structures with continuously changing period. However because of the limited position accuracy of any e-beam writer, it is not possible to generate the desired continuous variation of the grating period. There appear discrete displacement steps with deviations of some nanometers. We obtain some information about the parameters of the phase mask from the properties of written fiber gratings. A sequence of short fiber gratings is written in order to characterize the phase mask. We use a slit to expose a definite small part of the phase mask pattern during the writing process of each short fiber grating. The center wavelength of each fiber grating is analyzed.

Investigation of computer-generated diffractive beam shapers for the task of Gauss-to-ring laser beam transformation

To solve a special task of laser-beam material treatment, a new diffractive element was designed, fabricated and investigated, which converts a given single-mode CO2-laser beam into a ring-shaped intensity distribution in the working plane. This was accomplished by application of refined ray-tracing methods and an enhanced micro technology using e-beam generated masks. Relevant steps of calculation and manufacturing of these masks are outlined. First results of measured intensity distribution demonstrate a good conformity with results of computer simulations.

Development of a novel x-ray focusing technique using crystals with a two-dimensionally modulated surfaces

Theoretical investigations for obtaining x-ray point focusing by using crystals with two- dimensionally modulated surfaces are carried out. Based on the Bragg and Fresnel diffraction principles, formulae of modulated surfaces (structures) are derived for both flat and bent crystals for focusing x rays to micron or submicron size. It is found that elliptically shaped and linearly modulated structures are suitable for flat and cylindrically bent crystals, respectively. For the given Ti K(alpha) radiation and geometric parameters, Si (111) and InSb (111) reflections are used for the calculations of flat and bent crystals in terms of their focus characteristics, namely the focusing efficiency and the focus width. The influence of the distribution of the Bragg amplitude on flat and bent crystals is also discussed.

Nonlinear resonance effects on thin micro structured aluminum metal gratings by high power fs-laser pulses

We present measurements and numerical findings of resonance effects in aluminum metal gratings on fused silica substrates. The spectral characterization measurements of the gratings are done with white light in the wavelength range of 600nm up to 1600nm. Nonlinear effects have been studied with a fs-laser system at the resonance wavelength of the gratings. The metal layers of the gratings are 20nm to 40nm thick, the gratings period is 500nm or 1000nm and the narrow gaps between the metal stripes consist of about 8nm thick aluminum oxide stripes. These structures were produced with a special micro structuring process and coating technique. Numerical modeling shows that light can pass through these gratings for a characteristic resonance wavelength. For fs-pulses we found the transmission to decrease for increasing pulse energy.